Sensors are used to monitor the permissible levels of certain gases (e.g., hydrogen sulfide) in working environments. These sensors must be tested from time to time to make sure they are sufficiently sensitive to the presence of such gases. One test method involves the use of a cylinder containing a mix of gases including trace amounts of the gas or gases to be monitored. The gaseous mixture is released from the cylinder in the vicinity of the sensor to confirm that the sensor is working and that it is properly calibrated. To insure accurate test results, it is important that the mixture of gases in the cylinder remains stable over time.
In conventional cylinder designs, a valve with a thermal relief feature is connected to the cylinder to vent the cylinder in the event of excessive heat. One such valve 1, shown in FIG. 1, comprises a valve body 3 and check valve components 5 constructed of nickel-plated brass. The valve body 3 has a vent passage 7 containing a plug of solder 9 which melts when heated above a predetermined temperature. A significant number of cylinders contain reactive gases (e.g., hydrogen sulfide) that will react with copper. Since brass contains a significant amount of copper, it is undesirable to have such reactive gases come into contact with the brass in the relief valve 1. Although the valve body 3 and check valve components 5 are plated with nickel to shield the brass from the gas in the cylinder, small chips or holes in the plating can allow the gas to contact the brass. Any such contact can potentially change the concentration of the trace gases in the cylinder, which will adversely affect the test procedures discussed above.
There is a need, therefore, for an improved cylinder valve having a thermal relief feature, where the valve is designed to avoid any adverse affect on the stability of the gaseous mixture in the cylinder.